English

• In recent years, semicarbazones have become increasingly important as fine chemical intermediates^[1-3], which could also form complexes with the transition metal ions, producing amazing biological activities, such as anti-bacterial, antitumor, antivirus^[4-6].

  On the other hand, as one of the most important heterocycles, pyrazine is of significant interest in the pharmacological field^[7].Our previous work has shown that several Cu(Ⅱ)/Zn(Ⅱ) complexes with semicarba-zones bearing pyrazine unit have potential antitumor activities^[8-10].However, the investigation on the Ag(Ⅰ) complexes of such type of ligands are relatively scarce^[11].In fact, Ag(Ⅰ) complexes have been known for centuries, especially with respect to medical prop-erties^[12].Meanwhile, the polynuclear metal complexes are highly desired, because they usually possess higher biological activities than the mononuclear ones^[13-15].As the continuation of our work, the complex [Ag₂(HL)(NO₃)₂]_n (1) derived from 1-(3-ethylpyrazin-2-yl)ethylidene-4-phenylsemicarbazide (HL) has been synthesized and structural determined by single-crystal X-ray diffraction.In addition, the interactions of HL and 1 with ct-DNA have been studied by ethidium bromide (EB) fluorescence probe.

  1.   Experimental

  1.1   Materials and measurements

  Solvents and starting materials for synthesis were purchased commercially and used as received.The ligand HL was synthesized according to the reference^[10].Elemental analysis was carried out on an Elemental Vario EL analyzer.The IR spectra (ν=4 000~400 cm^-1) were determined by the KBr pressed disc method on a Bruker V70 FT-IR spectrophotometer.The UV spectra were recorded on a Purkinje General TU-1800 spectrophotometer.The interactions between the compounds and ct-DNA are measured using literature method^[9] via emission spectra on a Varian CARY Eclipse spectrophotometer.

  1.2   Preparation of complex 1

  The complex 1 was generated by reaction of the ligand HL (5 mmol) with AgNO₃ (molar ratio 1:2) in methanol solution (10 mL).Crystals suitable for X-ray diffraction analysis were obtained by evaporating the corresponding reaction solutions at room temperature.Colorless blocks.Anal.Calcd.for C₁₅H₁₇Ag₂N₇O₇(%):C 27.86, H 2.67, N 15.59; Found(%):C 28.91, H 2.75, N 15.73.FT-IR (cm^-1):ν(C=O) 1 685, ν(C=N) 1 600, ν(C=N)_pyrazine 1 537, ν₁(NO₃) 1 498, ν₄(NO₃) 1 382 and 1 311.

  1.3   X-ray crystallography

  The X-ray diffraction measurement for HL and complex 1 was performed on a Bruker SMART APEX Ⅱ CCD diffractometer equipped with a graphite monochromatized Mo Kα radiation (λ=0.071 073 nm) by using φ-ω scan mode.Semi-empirical absorption correction was applied to the intensity data using the SADABS program^[16].The structures were solved by direct methods and refined by full matrix least-square on F² using the SHELXTL-97 program^[17].All non-hydrogen atoms were refined anisotropically.All the H atoms were positioned geometrically and refined using a riding model.Details of the crystal parameters, data collection and refinements for complex is summarized in Table 1.

  表 1

  

  表 1  Crystal data and structure refinement for HL and complex 1

  Table 1.  Crystal data and structure refinement for HL and complex 1

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  	HL	1
  Empirical formula	C15H17N5O	C15H17Ag2N7O7
  Formula weight	283.34	623.10
  T/K	293(2)	296(2)
  Size/mm	0.20×0.18×0.08	0.25×0.22×0.19
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/n
  a/nm	0.739 28(15)	1.137 2(2)
  b/nm	1.428 5(3)	1.386 4(3)
  c/nm	1.529 5(3)	1.275 6(3)
  α/(°)	68.372(3)
  β/(°)	80.883(3)	105.618(3)
  γ/(°)	79.671(4)
  V/nm3	1.469 6(5)	1.937 0(7)
  Z	4	4
  Dc/(g·cm-3)	1.281	2.137
  Absorption coefficient/mm-1	0.085	2.079
  F(000)	600	1 224
  Reflection collected, Unique (Rint)	7 511, 5 151 (0.023 7)	9 458, 3 426 (0.021 4)
  Rint	0.024	0.021
  Data, restraint, parameter	5 151, 0, 381	3 426, 0, 281
  Goodness-of-fit (GOF) on F2	1.033	1.026
  R indices [I≥2σ(I)]	R1=0.057 0, wR2=0.126 2	R1=0.032 9, wR2=0.074 7
  R indices (all data)	R1=0.120 3, wR2=0.154 7	R1=0.040 9, wR2=0.078 8

  CCDC:1588866, HL; 1588867, 1.

  2.   Results and discussion

  2.1   Crystal structure description

  Selected bond distances and angles of HL and complex 1 are listed in Table 2.As shown in Fig. 1a, there are two independent molecules in the asymmetry unit of HL, in which the bond lengths of C9-O1 (0.122 6(3) nm) and C24-O2 (0.122 6(3) nm) are comparable to those of some reported semicarbazone ligands^[18].Pairs of intermolecular N-H…O hydrogen bonds link two molecules into a centrosymmetry dimer in the solid state (N4-H4…O2, with D…A distance being 0.286 9(3) nm, D-H…A angle being 162.0°; N9-H9…O1, with D…A distance being 0.294 0(3) nm, D-H…A angle being 162.0°).

  表 2

  

  表 2  Selected bond lengths (nm) and angles (°) of HL and complex 1

  Table 2.  Selected bond lengths (nm) and angles (°) of HL and complex 1

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  HL
  N4-C9	0.135 8(3)	C7-N3	0.127 3(3)	O1-C9	0.122 6(3)
  N9-C24	0.136 2(3)	C22-N8	0.128 6(3)	O2-C24	0.122 6(3)
  1
  Ag1-O1	0.241 9(3)	Ag1-N1	0.242 1(3)	Ag1-N3	0.238 3(3)
  Ag1-O2	0.238 7(3)	Ag1-O6	0.266 7(3)	Ag2-O5	0.253 4(4)
  Ag2-O3ⅰ	0.229 0(3)	Ag2-N2	0.225 1(3)	Ag2-O4	0.279 9(4)
  N3-Ag1-O1	67.11(10)	O1-Ag1-N1	134.08(10)	O2-Ag1-O1	134.45(11)
  N3-Ag1-O2	145.68(11)	O2-Ag1-N1	85.97(10)	N3-Ag1-N1	67.02(10)
  N2-Ag2-O5	103.99(13)	N2-Ag2-O3ⅰ	155.04(13)	O3ⅰ-Ag2-O5	82.88(13)
  N3-Ag1-06	124.40(11)	O2-Ag1-O6	81.52(11)	O1-Ag1-O6	102.53(10)
  O1-Ag1-O6	102.53(10)	N1-Ag1-O6	104.69(12)	N2-Ag2-O4	115.73(11)
  O3-Ag2-O4	47.71(11)	O5-Ag2-O4	128.40(12)
  Symmetry codes:ⅰx+1, y, z

  As illustrated in Fig. 1b, the asymmetry unit of complex 1 contains two independent Ag(Ⅰ) ions.One of them is surrounded by one HL ligand with ON₂ donor set, two O atoms from two nitrate anions, giving a distorted square pyramid coordination geometry; while its counterpart with a planar square coordination geometry is coordinated with three O atoms from two adjacent nitrate anions and terminal pyrazine N4 atom of HL.It is worth noting that O4 and O6 atoms also participate coordination, since the bond lengths of Ag1-O6 (0.266 7(3) nm) and Ag2-O4 (0.279 9(4) nm) are within the scope of normal inspection and matches with the Ag-O bond in some Ag(Ⅰ) complexes in the literature^[19].The neighboring Ag(Ⅰ) ions are linked by two types of nitrate ions into a 1D chain along a axis (in the case of N7/O5-O7, Fig. 1c), and a 1D zig-zag chain along c axis (in the case of N6/O2-O4, Fig. 1d), respectively, thus giving a 2D framework paralleling to (101) plane (Fig. 1e).In the crystal, intermolecular N-H…O hydrogen bonds between the semicarbazone ligands and nitrate anions are also presented (N5-H5…O7^ⅳ, with D…A distance being 0.289 5(5) nm, D-H…A angle being 162.0°; N4-H4…O6^ⅳ, with D…A distance being 0.307 1(5) nm, D-H…A angle being 165.0°, Symmetry codes:^ⅳx, y, z+1).

  图 1

  

  图 1  Diamond drawing of HL (a) and 1 (b) with 30% thermal ellipsoids; (c) Chain-like structure along a axis in complex 1; (d) Zig-zag chain-like structure along c axis in complex 1; (e) Extended 2D framework structure in complex 1

  Figure 1.  Diamond drawing of HL (a) and 1 (b) with 30% thermal ellipsoids; (c) Chain-like structure along a axis in complex 1; (d) Zig-zag chain-like structure along c axis in complex 1; (e) Extended 2D framework structure in complex 1

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  2.2   IR spectra

  The infrared spectral bands most useful for deter-mining the mode of coordination of the semicarbazone ligand are the ν(C=O), ν(C=N), and ν(C=N)_pyrizine vibrations.As our previous work shows, such three bands of the ligand HL are at 1 705, 1 609 and 1 595 cm^{-1 [10]}, while they shift to 1 685, 1 600 and 1 537 cm^-1 in the complex 1, respectively, indicating that the carbonyl O, imine N and pyrizine N atoms take part in the coordination^[8].In addition, the intense absorption bands in the spectra of complex 1 associated with the asymmetric stretching appear at 1 382, 1 311 cm^-1 (ν₄) and 1 498 cm^-1 (ν₁), clearly establishing the existence of monodentate and bidentate NO₃^- ligands^{[8, 10, 20]}.It is in accordance with the crystal structure study.

  2.3   UV spectra

  The UV spectra of complex 1 in methanol solution (concentration:10 μmol·L^-1) were measured at room temperature (Fig. 2).The spectra of HL features one main band located around 275 nm (ε=6 929 L·moL^-1·cm^-1) and a shoulder at 282 nm (ε=6 567 L·moL^-1· cm^-1), which could be assigned to characteristic π-π^* transition of benzene and pyrazine units, respectively^[10].In complex 1, such two peaks are merged to 291 nm (ε=3 813 L·moL^-1·cm^-1) with concomitant hypochromic effect, confirming the coordination of ligand HL in complex 1.

  图 2

  

  图 2  UV spectra of the ligand HL and complex 1 in the methanol solution at room temperature

  Figure 2.  UV spectra of the ligand HL and complex 1 in the methanol solution at room temperature

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  2.4   EB-DNA binding study by fluorescence spectrum

  It is well known that EB can intercalate nonspe-cifically into DNA, which causes it to fluoresce strongly.Competitive binding of other drugs to DNA and EB will result in displacement of bound EB and a decrease in the fluorescence intensity^[9].As shown in Fig. 3, the fluorescence intensities of EB bound to ct-DNA at about 600 nm show remarkable decreasing trends with the increasing concentration of the tested samples, indicating that some EB molecules are exchanged by the tested compounds.The quenching of EB bound to DNA by the compounds is in agreement with the linear Stern-Volmer equation:I₀/I=1+K_sqr^[8], where I₀ and I represent the fluorescence intensities in the absence and presence of quencher, respectively, K_sq is the linear Stern-Volmer quenching constant, r is the ratio of the concentration of quencher and DNA.In the quenching plots of I₀/I versus r, K_sq values are given by the slopes.The K_sq values of complex 1 is tested to be 0.781, which is much higher than that of the ligand HL (0.363).The results indicate that inter-actions of the complex 1 with DNA are stronger than that of the ligand HL.This is probably due to the structure rigidity and metal-ligand synergism effect of the complex 1^[8-9].In addition, 2D framework structure of the complex 1 may be also responsible for its DNA interaction ability in some content.

  图 3

  

  图 3  Emission spectra of EB-DNA system in the presence of HL (a) and complex 1 (b), respectively

  Figure 3.  Emission spectra of EB-DNA system in the presence of HL (a) and complex 1 (b), respectively

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• 

  Figure 1  Diamond drawing of HL (a) and 1 (b) with 30% thermal ellipsoids; (c) Chain-like structure along a axis in complex 1; (d) Zig-zag chain-like structure along c axis in complex 1; (e) Extended 2D framework structure in complex 1

  Symmetry codes:^ⅰx+1, y, z; ^ⅱ-x,-y,-z+1;^ⅲx-1, y, z; ^ⅳx, y, z+1 for 1

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  Figure 2  UV spectra of the ligand HL and complex 1 in the methanol solution at room temperature

  c=10 μmol·L^-1

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  Figure 3  Emission spectra of EB-DNA system in the presence of HL (a) and complex 1 (b), respectively

  Arrow shows the fluorescence intensities change of EB-DNA system upon increasing tested complex concentration; Inset:plot of I₀/I versus r

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  Table 1.  Crystal data and structure refinement for HL and complex 1

  	HL	1
  Empirical formula	C15H17N5O	C15H17Ag2N7O7
  Formula weight	283.34	623.10
  T/K	293(2)	296(2)
  Size/mm	0.20×0.18×0.08	0.25×0.22×0.19
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/n
  a/nm	0.739 28(15)	1.137 2(2)
  b/nm	1.428 5(3)	1.386 4(3)
  c/nm	1.529 5(3)	1.275 6(3)
  α/(°)	68.372(3)
  β/(°)	80.883(3)	105.618(3)
  γ/(°)	79.671(4)
  V/nm3	1.469 6(5)	1.937 0(7)
  Z	4	4
  Dc/(g·cm-3)	1.281	2.137
  Absorption coefficient/mm-1	0.085	2.079
  F(000)	600	1 224
  Reflection collected, Unique (Rint)	7 511, 5 151 (0.023 7)	9 458, 3 426 (0.021 4)
  Rint	0.024	0.021
  Data, restraint, parameter	5 151, 0, 381	3 426, 0, 281
  Goodness-of-fit (GOF) on F2	1.033	1.026
  R indices [I≥2σ(I)]	R1=0.057 0, wR2=0.126 2	R1=0.032 9, wR2=0.074 7
  R indices (all data)	R1=0.120 3, wR2=0.154 7	R1=0.040 9, wR2=0.078 8

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  Table 2.  Selected bond lengths (nm) and angles (°) of HL and complex 1

  HL
  N4-C9	0.135 8(3)	C7-N3	0.127 3(3)	O1-C9	0.122 6(3)
  N9-C24	0.136 2(3)	C22-N8	0.128 6(3)	O2-C24	0.122 6(3)
  1
  Ag1-O1	0.241 9(3)	Ag1-N1	0.242 1(3)	Ag1-N3	0.238 3(3)
  Ag1-O2	0.238 7(3)	Ag1-O6	0.266 7(3)	Ag2-O5	0.253 4(4)
  Ag2-O3ⅰ	0.229 0(3)	Ag2-N2	0.225 1(3)	Ag2-O4	0.279 9(4)
  N3-Ag1-O1	67.11(10)	O1-Ag1-N1	134.08(10)	O2-Ag1-O1	134.45(11)
  N3-Ag1-O2	145.68(11)	O2-Ag1-N1	85.97(10)	N3-Ag1-N1	67.02(10)
  N2-Ag2-O5	103.99(13)	N2-Ag2-O3ⅰ	155.04(13)	O3ⅰ-Ag2-O5	82.88(13)
  N3-Ag1-06	124.40(11)	O2-Ag1-O6	81.52(11)	O1-Ag1-O6	102.53(10)
  O1-Ag1-O6	102.53(10)	N1-Ag1-O6	104.69(12)	N2-Ag2-O4	115.73(11)
  O3-Ag2-O4	47.71(11)	O5-Ag2-O4	128.40(12)
  Symmetry codes:ⅰx+1, y, z

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•